It has been a generally practiced procedure to indicate a valid period or a date of production as a means of indicating the length of time a product is effective, either on the product itself or on the packaging of, for instance, food products such as frozen food products, refrigerated food products, perishable food products, chemical products such as paints, glues and the like. However, the actual valid period for these products changes greatly depending upon how they are stored after they are produced; specifically, at what temperature and for how long. For instance, if such a product is stored in a freezer or refrigerator, the valid period is greatly lengthened compared to a case in which it is stored at room temperature.
This means that in the conventional method, in which either an absolute "best before" date is set for a product or the date of production is indicated without taking into consideration the storage temperature conditions or the length of storage at specific temperatures, the actual valid period can not be accurately indicated.
As a technology that addresses the problem, a tape constituted by combining a first tape provided with a chemical material layer on one surface and a second tape provided with a chemical material layer on one surface which, upon being placed in contact with the chemical material layer on the first tape, generates a chemical reaction to indicate a temperature/time integrated value starting from the time when the two chemical material layers come in contact with each other, has been proposed, although the technology has not yet become publicly known.
Through this technology, the point in time at which the chemical material layers on the two tapes come in contact with each other is set as the starting point for time measurement, and the temperature/time integrated value which is determined by the product of the time elapsing from the starting point for time measurement and the history of temperatures under which the product has been stored during that period of time can be indicated with color. Consequently, the actual valid period can be indicated extremely accurately.
However, the technology for placing two different types of tape in contact with each other, which is essential for the application of this technology, has not yet been developed.